This invention relates to a method and an operating circuit both for xenon-metal halide lamps. More particularly, the invention relates to a method and an operating circuit that provide for direct current signals having a ripple imposed thereon for acoustically operating a xenon-metal halide lamp containing a high pressure of xenon that yields a light source which is particularly suited for automotive applications.
U.S. patent application Ser. No. 157,436 filed Feb. 18, 1988 of R. S. Bergman et al., assigned to the same assignee as the present invention and herein incorporated by reference, discloses a xenon-metal halide lamp particularly suited for automotive applications. The xenon-metal halide lamp provides improved efficiency and longer life relative to incandescent lamps while a high pressure of xenon gas primarily achieves instant light capabilities making such a lamp particularly suited for automotive applications.
U.S. patent application Ser. No. 320,736 filed Mar. 8, 1989 of G. R. Allen et al., assigned to the same assignee as the present invention, and herein incorporated by reference, discloses an AC ballast circuit for acoustically operating xenon-metal halide lamps to provide a straight arc condition between the electrodes of the xenon-metal halide lamp. The straight arc provides for a point-like source that is beneficial for optical purposes and also prevents the arc from touching the walls of the lamp so as not to reduce the life of the lamp. Further, in a preferred embodiment, frequency modulation is incorporated to broaden the desired operational frequency bands that provide for the straight arc.
Although the AC ballast circuit and method of operating of Ser. No. 320,736 serves well the needs of the xenon-metal halide lamp, it is desired that the circuit complexity be reduced in order to correspondingly reduce the cost and size of the ballast circuit so as to allow the ballast circuit to be more easily integrated into the automotive housing. A reduction could be provided by allowing the ballast circuit to be directly connected to the DC source of excitation (battery) of the automobile and thereby eliminate the associated circuit components of the DC/AC converter.
As discussed in U.S. patent application Ser. No. 157,360 of R. L. Hansler et al., assigned to the same assignee as the present invention and herein incorporated by reference, lamps having a metal halide ingredient, that are operated from a DC source of excitation typically experience the effects of cataphoresis which cause the halides of a metal halide lamp to be moved or swept into the end regions of the lamp so as not to contribute to providing the desired illumination of such a lamp. This cataphoresis effect is particularly noticed when the light source is horizontally arranged such as in automotive applications. It is desired that means be provided to allow for xenon-metal halide lamps to be operated from a DC source without suffering the accompanying disadvantageous effects of cataphoresis typically encountered for such DC lamp operation. In addition, it is desired that such DC operation provide for a straighten arc between the electrodes of the xenon-metal halide lamp.
Accordingly, it is an object of the present invention to provide a circuit connected to a DC source for operation of a xenon-metal halide lamp.
It is a further object to provide a method of operating a xenon-metal halide lamp in a DC manner without the disadvantageous effects of cataphoresis while at the same time providing a straight arc between the electrodes of the xenon-metal halide lamp.
It is a further object of the present invention to provide for a DC circuit that allows for an overall reduction in size relative to AC ballast type circuits employed for xenon-metal halide lamps.
Still further, it is an object of the present invention to provide for improvements that enhance the illumination patterns yielded by xenon-metal halide lamps and made available for automotive applications.